To improve fuel economy, fuel delivery to an engine in a hybrid or conventional powertrain vehicle may be deactivated during vehicle deceleration and vehicle stops. During deceleration, the engine, electric motor/generator (EMG), and air conditioner (AC) may be back driven by the vehicle wheels. When fuel delivery to the engine is deactivated, the EMG may recharge an energy storage device (ESD). The drag of the engine, EMG, and AC may result in increased negative drive-axle torque. Negative wheel slip may occur when the force required to back drive the engine, EMG, and AC becomes greater than the frictional force between the road and the driven wheels.
Positive wheel slip may occur when the operator aggressively drives the vehicle. When the vehicle is accelerated, the force of the positive drive-axle torque may become greater than the frictional force between the road and the drive wheels. In such case, positive wheel slip occurs. When aggressively driving the vehicle, the operator may disable the vehicle traction control system.
Traditionally, wheel slip is detected by the anti-lock braking system (ABS) and by the traction control system. In the traditional system, however, wheel slip is not controlled when the brakes are not applied or when the traction control system is disabled.